Composite metal and articles thereof



United States Patent O COMPOSITE METAL AND ARTICLES THEREOF John JamesRussell, Des Plaines, William A. Beck, Itasca, and Jack Kollman,Chicago, Ill., assignors to Ekco Products Company, Chicago, Ill., acorporation of Illinois No Drawing. Application December 18, 1952,Serial No. 326,765

11 Claims. (Cl. 204-37) This invention relates to a composite metallicstructure which may be fabricated into shaped metallic articles havingouter surfaces rendered highly absorptive to radiant energy whileretaining a relative high resistance to corrosion.

An object of our invention is the PIOViSlOll of a simple, direct, andthoroughly practical process for chemically treating tin and tin alloysby anodic treatment in various electrolytes, in order that the outwardsurfaces of the tin and tin alloys exhibit excellent heat absorption.

Another object of our invention is to provide a fabricated article whichhas been anodically treated wherein the surfaces of said article shallbe characterized by excellent heat absorption, high abrasion resistance,and continuity and uniformity of the treated tin or tin alloy surfaces.

Another object of our invention is the provision that the heat absorbingcoating produced by the chemical process for treating the surfaces oftin or tin alloy shall be adherent and sufficiently ductile in orderthat articles may be fabricated by mechanical, drawing, forming and/orbending operations.

Another object of our invention is the provision that a shaped compositesheet metal fabricated article, such as a baking pan or a process tray,possess outer and inner surfaces having relatively similar heatabsorbing properties.

Other objects and advantages of this invention will be made moreapparent as this description proceeds.

In the art of baking, the pans heretofore used have been constructed ofsheets having surfaces of tin or tin alloys. An example of such a sheetis tin plate in which the base material is steel. Of course, other tincoated sheets in which the base material is a material such as copperand brass have been used.

Hot dipped tin plate is generally considered to be steel base metalcoated on its exterior surfaces with metallic tin wherein intermediateto the tin and steel interface, an iron-tin alloy composition is formed.Tin plate may be formed by either the conventional method of hot dippingor by electrolytically depositing the tin on the surface. The coatingweight of the tin is usually specified in pounds of tin per basis box,or in grams of tin per square metre of tin. The conversion of weight perbasis box to linearv thickness depends on an assumed density which iscompensated by the fact that in the hot dipped process there is agreater proportional thickness of the alloy layer formed than in theelectrodeposited process.

It is generally accepted that one pound per basis box is equivalent to0.0000606 inch thick of tin on each face ofthe tin plate. One andone-half pounds tin plate is generally assumed to be about 0.0000909inch thick. The proportional ratio between thickness and total weight ofa tin plate can be approximated from the relative propor-' tion abovementioned for hot dipped tin plate.

In the case of the electrolytic process, tin plate thickness coatings of8 ounces to ounces per basis box is "ice generally accepted, althoughsome applications of tin plate may use less than 1 /2 pounds butgenerally not greater than 1 /2 pounds per basis box.

In the present invention, the anodic chemical treatment of tin plate isprimarily directed toward tin plate having the tin content greater thanabout 1% pounds per basis box which would primarily be adaptable to hotdipped tin plate, although the scope of the invention is also applicableto electrolytic tin plated articles.

' The formation of oxide films of tin, as heretofore known may beproduced by converting the tin into the oxide by subjecting the tinlayer under oxidizing conditions, such as air, at elevated temperatures.Thisproeess of subjecting the tin to oxidizing conditions at elevatedtemperatures, has been conventionally used by the baking industry inconverting the tin layers to oxides of tin by placing formed articles oftin plate in baking ovens at an elevated temperature of approximately440 to 425 F. for a period in excess of about 4 to 12 hours. This hasbeen conventionally called the burning in or burning out proc-,

ess, wherein shaped articles, such as baking pans, acquire a colorranging from the interference films of light iridescent hues rangingin-color from yellow, blue through greens, and subsequently a surfacecoating having a degree of coloring may be obtained depending upon thechemical and physical characteristics of the tin plate surface inconjunction with variable atmospheric conditions during the burning incycle.

It has been found that these oxide films possess wide variations incolor characteristics; and it is highly desirable to eliminate thesewide variations in order to produce uniform and consistent bread crustcolor. In addition, the tin-iron alloy intermediate to the tin layer andthe steel base-is brittle and less corrosion resistant than the tinlayer. Normally, the tin-iron alloy is substantially increased withrespect to the available metallic tin layer by these lengthy burning inoperations; and it is highly desirous to reduce the burning in time toabout 30 minutes to one hour. The baking industry finds it economicallyundesirable to subject the bread pan to long periods of burning inbecause it ties up the baking ovens, bakmg pans, as well as personnel ina non-productive operatron and also produces variable results.

In addition, the temperature controls of the oven may vary considerably.It has been found that oventemperatures generally will exceed themelting point of the tin layer on the surface; as a result the tin-ironalloy layer will be substantially increased and the baking pans will besubsequently destroyed by the evaporation and decomposition of tinsurface layer. Also complete uniformity of crust color of bread is adefinite sales factor,

. and a baker who can produce a uniform loaf of bread in the initialbaking operation without the conventional burning in of a new set ofpans, will increase his production capacity.

In addition, the adherence of oxide of tin, by the conventionalburning-in method, has been found to be chemical and physicalcomposition of the surface layers I, of the tin will produce variationin adherence of the oxide coatings.

Therefore, it is desirous to secure oxide coating formation having heatabsorption which is uniform and reproducible and at the same time tohave a surface ox-ide coating which will not tend to reflect heat energythat impinges upon the surface of objects made I of tin plate.

the curing of the film is simultaneous with the final step of theformation of the green oxide of tin coating. In the formation of' thegreen oxide of tin coating, a tin plated article is subjected, to an,anodic electrolytic treatment,

as hereinafter described, to form a'tuniform meta s'table tin oxidecoating 'on tin plate, and thereafter to the article 15 applied anorganic releasing film and the resulting product is heated to anelevated temperature :to simultaneously convert the meta-stable tinoxide, which is bel1eved to be amixture of stannous oxideand stannicoxide, to the tenaciously adherent green oxide while curing the organicfi1m. v

The anodic chemical process of the present invention for treating tin;surface layers will substantiallyireduce the heretofore mentionedfobjections; and in addition, willsubstantially remove objectionablecarbonaceous deposits within theporous tinplate, such as g'rea'seandoil.

In addition, the anodic chemical process will tend to seal the pores ofthe tin plate, thus reducing the tendency toward porosity corrosion, Inthe chemical anodictreatmerit of tin plate, or products of variousshapesiand coi'lfigurations, the tinplate layer iselectrolyticallytneated by using one or more of the plates, articles orproducts as the anodes in an electrolytic bath.

Y The e ectrolytic bath contains at least one "or more, or combinationsthereof, of a reagent consisting of substantial amounts of 1) polybasicorganic acids, such as citric acid, picric, tartaric acid, oxalic acid,malic acid, maleic acid, and succinicacidgand (2) monobasic organicacids, such as acetic acid, lactic acid, propionic acid,benzenesulphonic acid, trichloroacetic acid, and salicylic acid; and.(3) non-oxidizing inorganic acids, such asphosphoric acids, boric,molybdic, tungstic, andhydrofluoric acids;and (4) aqueous soluble salts,such as the alkali metal and/or alkaline earth salts of the abovementioned organic acids or inorganic acids and other metallic saltcompositions, It has been found that combinations offthe reagentsinaqueous solutions may be employed such as: phosphoric acidcombined withcitric acid; phosphoric acid combined withsodium phosphate; phosphoricacid combined with oxalic acid, acetic acid combined with sodiumcitrate; and sodium phosphate ,cbmbinedwith sodium citrate. The scope ofthis invention shall not be limited to the chemical composition of thereagents. v c t In the anodic, treatment of tin-plated articlesandprodnets in the electrolyte, it has been found advantageous tomaintaincurrent densities rangingfrom about 4 amp'jere'sper square footto about 60 amperesper square foot'of tin surfaces undergoing treatmenttogether with a "solution temperature of at least 50 Q; and usuallymore, up to the maximum temperature which fallsbelow the boiling pointof the solution so as not to cause ex ces'sive evaporation.- Under thechemical conditions specified together with time of immersion, it ispossible to obtain a uniform meta-stable tin oxide coating on tin platehaving thickness ranging from about 3 toS micr'oinches to almostcomplete conversion of the free tih'su'rface. Lower temperatures for theelectrolyte, -ineluding room temperature, have been foundsuitable forconverting at least part of the tin layer to uniform metastable "tinoxide coating, and withthese lower temperaturesthe higher currentdensities areutilized, y c

It is preferred to employ an electrolyte which by weight consists of atleast 0.5% up to about 30% of the reagents wherein the pH of thesolution'may be adjusted if necessary, with the corresponding acid orcombination of other acids-so that the pH range may vary 'fro'rn 'ahout'apH- of 2 to a pH of about 8; and the remaining parts needed to form100% by weight being substantially of water. It has been found that theoxide of tin is more readily formed when the pH is within the range ofabout two and one-half (2 /2) to five (5).

One or more other tin plated articles, such as tin formed baking pans,are made the anodes of the electrolytic solution and are subject toanodic treatment while maintaining a preferred solution bath temperatureof about-8O to 98 degrees C. and a current density of about 20 to 40amperes per square foot. The cathode may consist of shapedmetallic'compositions, such stainless steel, or a lead lined tank may beused as theca'thode. Under such temperature-conditions, the solution isfound to remain stable to the extent that the reagent is substantiallyretained in the bath with respect to weight percentage. removed from thesolution.

A uniformgmeta-stable oxide of tin may be imparted to the tin surface ina short interval of time. Th'e tiine of immersion togethe-rwith solutionconditions arenaportant factors with regard to the amount of meta-statuetin oxide formed by the anodic treatment. It' hasheen found thatinterference films of about 2 to 6 ier'aiaehe" thick may be formedbyanodic treatment, in the e ect'rolyte'for an interval of timecorresponding to a few s'econds, In addition, it has been found that ina matter of minutes'the entire tin layer can be converted intoa inetastable oxide of tin,

As "examples'of other exceptionally stable-conductiveand highlyeffective electrolyte'solut ions, [and the relalted operating conditionswhich are employed for rapidly obtaining a uniform continuousmeta-stable oxide of an; the following treatments may be'employed:

T r eatme nt A Percent: Electrolyte Total Weight 'Of BathDisodium'Hydi-ogen Phosphate NMHP 04 "3-15 Ortho Phosphoric Acid HaPO-i(%); -2-5 "re'r'n'ainiiig p'a'nts'needed with the-above to total 100%(percent) by weight, being substantially Water.

--re1naining--- parts needed f above --to total 100 percent by weight,beingsubstantially water.

Any rsm'ainia pam needed 'with theabove mm: 100% (percent) by weight,being substantially water.

The reagent" is not substantially chemically Any remaining parts neededwith the above to total 100% (percent) by weight, being substantiallywater.

Bath temperature 80-100" C. Minimum current density 15 amperes per sq.foot. Time of immersion 30120 seconds.

Treatment E Percent Electrolyte Total Weight of Bath Sodium TartrateN82C4H40e-2H20 3-30 Disodium Hydrogen Phosphate 2-10 Any remaining partsneeded with the above to total 100 percent by weight, beingsubstantially Water.

Bath temperature 90-100 C. Minimum current density 15 amperes per sq.foot. Time of. immersion 15 to 120 seconds.

Treatment F Percent Electrolyte Total Weight of Bath Disodium hydrogenphosphate 3-30 Any remaining parts needed with the above to total 100percent by weight, being substantially Water.

Bath temperature 80-100" C. Minimum current density 20 amperes per sq.foot. Time of immersion 30 to 120 seconds.

Treatment G Percent Electrolyte Total Weight of Bath Sodium OxalateN3zCzO4 2-15 Phosphoric Acid HsP O4 (85%) 3-10 Any remaining partsneeded with the above to total 100% (percent) by weight, beingsubstantially water.

' Any remaining parts needed with the above to total 100% (percent) byweight, being substantially water.

Bath temperature -100 C. Minimum current density 20 amperes per sq.fioot. Time of immersion 60 to seconds.

An excellent meta-stable oxide layer of tin was obtained on tin plate inthe instance of using Treatment A through Treatment H, inclusive. Thescope of the invention should not be bound by any such quality ofchemical composition, nor by the specific proportions of acids, salts,and water given in the several illustrative examples of the treatment.

The treated tin plate articles are then removed from the electrolytebath and rinsed with water in order to remove any occluded salts. Thetreated articles are then dried at slightly elevated temperature inorder to remove the adherent water. The treated tin plate articles uponwhich the meta-stable oxide of tin is deposited or formed thereon aresubjected, after the application of the organic bread releasing film, ashereinafter described, to an oxide conversion temperature conditionwhereby the metastable oxide of tin coating is then converted atelevated temperatures to form the stable, tenaciously adherent greenoxide of tin coating while at the same time curing the organic film. Forconversion of the tin oxide temperatures as low as 150 C. can be used,but since the time required for temperatures below about C. are quitelong, it is preferred that the temperature range be from about 190 C. toabout 230 C. Of course, where the tin coating has been entirelyconverted to meta-stable tin oxide in the electrolytic treatment,temperatures above 230 C., for example as high as 400 (3., can beutilized when the entire oxide of all of the oxide of tin coatings havebeen coated with the organic film because an inner layer of tin need notbe relied upon for corrosion prevention. This heating step in theprocess may be obtained by direct heat application to formulate theoxide conversion.

The formation of the meta-stable oxide of tin under electrolytictreatment will vary depending upon solution compositions and conditions.Since tin exhibits amphoteric properties, it has been noted that themeta-stable oxide of tin may be formed under alkaline conditions,although the electrolytic step in the process is preferred under acidconditions (pH from 2% to 5), successful results have been obtainedunder alkaline conditions. In addition it has been noted that theformation of the meta-stable oxide of tin may be formed first byimmersion in an alkaline medium and then subsequently into an acidmedium or combinations thereof.

The meta-stable tin oxide coating exhibits the interference color on thesurface of tin, and when the thickness of the meta-stable tin oxideappears to approach about seven (7) microinches, capacity of the filmbegins to occur. By varying the time of immersion in the electrolyticbath, the blue-black meta-stable tin oxide is formed and at about 2 to10 minutes immersion time under the aforementioned conditions it appearsthat the tin layer may be converted totally into the meta-stable tinoxide. Therefore, it is desirable to reduce the time of electrolyticimmersion to about fifteen to ninety (15-90) seconds in order to retaina relatively high percentage of free tin.

The time interval for electrolytic immersion will vary depending uponsolution conditions. By reducing the current density and/or thetemperature as well as the concentration of the reagent, it is possibleto increase substantially the time interval of immersion of the tinlayer in the electrolytic bath. Therefore, the scope of the inventionshall not be bound by the specific time interval of immersion.

The meta-stable tin oxide exhibits various transitional interferencecolors approaching the blueblack. The blueblack is then considered theend point of opacity; and it has been found that the interference colorsof red, purple, green, and blue-green will form, upon subjecting thethen treated meta-stable tin oxide coating to conversion.

seesaw 7 conditions at"elevated'temperature; the stable tin oxidecoating-exhibiting a uniform olive green color which is believed to be amixture of stannous oxide and st-annic oxide.

The novel chemical treatment described in this process may be furtheraccomplished by'using a fused salt, such as diso'diu mhydrogen phosphatedodeca-hydrate or magnesium phosphate, as the electrolyte, orcombinations thereof. One may consider. this to be substantially anaqueous solution when the temperature of the electrolyte bath is within80 to 100 C; We have found that satisfactory results can beobtained byholding the bath temperature between 80 and 90 C. operating at a minimumcurrent density of about 15 amperes for a square foot for a period oftime of about 20 to 90 seconds. When-the current density is held atabout 30 amperes per square'foot, it-has'been found that the voltagedropped to about 2 volts.

lt shall be noted that the process as described may be applied to tinplate in sheet formwherein the anode may be a sheet of tin. plate or ashaped article. Thetin plate or 'com-p'osite sheet structure may bechemically treated by'the treatments aforementioned and then fabricatedinto shaped articles.

An alternative method of manufacturing these shaped articles, havingheat absorbing oxide coatings formed thereon, may be accomplished byforming the tin plate and then subjecting the formed shape to theanodicchemical treatment by immersing the shaped article in theelectrolyte. The cathode generally will have to be shaped into variousforms in order thata uniform oxide of tin coating may be formed on thetin layer; especially wherein the surfaces of the shaped article aredeeply formed.

Another step of this invention comprises the coating of the oxide of tincoatings with a stable high temperature organic bread .releasing'film.The organic bread releasing film may be applied prior to the finaltemperature conversion step so that the meta-stable tin oxide is coatedwith a stable high temperature organic bread releasing film.

Examples of this film are films of the polyorgano siloxanes, alsogenerically known as silicones, in which silicon atoms are linked toother silicon atoms by means of oxygen atoms, and in which organicgroups are attached to silicon atoms. Examples of organic groups arealiphatic radicals,-especiallyalkyl groups, and aryl radicals such asthe phenol and naphthyl groups. Other examples of organic groups includethe alkaryl radicals, such as tolyl and xylyl radicals, andarlkylradicals, such as benzyl radical. In all of these organic groups theremay be substitution of'other atoms or radicals, such as chlorine,hydroxyl and bromine; for example, the organic groups may bechloromethyl, chlorophenyl, brornophenyl, hydroxyethyl, andhydroxyphenyl.

These polyorgano siloxanes may be the silicone resins, such 'asthe alkylsilicones and alkyl aryl silicones in whicha considerable proportion ofthe siloxane units of thepolymer contain about one organic group persilicon atom, and in which another portion of the siloxane unitscontaintwo organic groups, preferably in which one of the groups is analkyl radical, such as methyl radical, while the'other group is an arylradical, such as the phenyl group. 'These resins are known as alkyl arylsilicones when at least a portion of the siloxaneunits contain both anallryl group and an aryl group. In addition to the silicone'resins,viscous silicone oils. may be used.

Other examples of organic bread releasing films are the.polyorganos'ilanes, suchas disclosed in U. S. Patent 2,606,837, grantedto Harold A. Clark on August 12, l95-2,'in which these polymers aremerely applied to commercial b'read pans of the prior art, that is, panswhich did not contain the uniform continuous tenaciouslyadhe'r'entmeta-stable tin-oxide coatings of this invention,

and thus did not contain the uniform continuous tenac iouslyadherentopaque green oxide of tin coatings that 8 are obtained in carrying-outthecomplete rocessor this invention. 7

A further example of theorganic is'fihn of'a polymer of afluorine-containing derivative of ethylene,

such as polytetrafluoroethylene, polytrifluoromonochloroa ethylene,polydifluorodichloroethylene, and polyvinylidcne fluoride. adequatelydescribed in the published literature; for example, a process ofpreparing the polymer of tetrafluoroethylene is described in U. S.Patent 2,534,058, granted to M. M. Renfrew on December 12, 1950, inwhich an" aqueous dispersion or-suspension of the polymer isob tained.Such aqueous colloidal dispersionscan be concentrated by the methoddisclosed in U. S. Patent 2,478,229 granted to K. L. Berry on August 9,1949.

The polymers of the other fluorine-containing derivatives of ethylenecan be made by similar processes or by other methods, for example, thepolymerization ofvinylidene fluoride is described in Industrial andEngineering Chemistry, volume 41, page 71 (January 1949), and thepreparation of polymers of trifluoromonochloroethylene .is described inBritish Patent 465,520, granted May 3, 1937, to I. G. Farbenindustn'e A.G.

The aqueous colloidal dispersions of these polymers,

can be stabilized by the use of nonionic and anionic dispersing agents,for example, alkyl aryl polyether al- The stabilizing effect of thesedispersing agents is described in Industrial and cohol and sodium laurylsulphate.

Engineering Chemistry, volume 44, pages 1800-5 (August 1952).

A still further example of organic bread-releasing films are the filmsobtained using allyl polymers, piererably resinous allyl polymer. Theallyl resinsare broad ly polybasic acid-alcohol esters and polymersthereof, which may be prepared by various methods such as by esterifyinga polybasic acid and an allyl alcohol with p or without additionalalcohols to produce the mixed ester desired. These compounds may also beprepared from the corresponding acid chlorides of the acid esters, suchas allyl succinyl chloride. This acid chloride may be reacted withpolyhydroxy compounds to form the desired products. Alternately, thepolyhydi-ic esters of a polybasic acid such as glycol dihydrogendiphthalate or other partial esters of a polyhydric alcohol and apolybasic acid may be esterified with allyl alcohol or;

with a mixture of such alcohols as methallyl, allyl and crotyl alcoholsand the like. Various catalysts are gen= erally used at elevatedtemperatures.

By polymerization of the compounds formed, it is possible to secure awide range of intermediate fusible.

polymers. These fusible polymers may be prepared by polymerizing theunsaturated compounds until substantial polymerization has occurred andinterrupting the polymerization before the'polymer is converted into agel.

It is found that when polymerization of these materials is initiated, afusible polymer is primarily formed. This fusible allyl component may bepartially or totally dissolved in such organic solvents as acetone,dioxane,

benzene, toluene, chloroform, acetates and the like.

The resinous components are dispersed in the respective solvents andmixed,'-whereupon the combination is ap-' plied to tin plated bakingpanshaving meta-stable tin oxide coatings, dried for 30 minutesand cured,for ex-' ample, at 425 F. for one hour, which simultaneously convertsthe tin oxide to green oxide of tin. In making comparative tests of thereleasing properties of bake goods, a standard baking dough mixturehaving a relatively highsugar content and a low shortening content isused 'to determine the relative releasing properties of variou coatings.By introducing this minor proportion of the incompatible resin componentto the allyl resin polymer, the number of releases can be greatlyincreased. In addition, we have found that in many instances, curedallyl resinous coating will not'frelease. this. standard 1 dougnnixtureeven on the first'bake'; while in Processes for preparing .thesepolymers are of bake pans having a film of the resinous compositionemployed in the present invention, we have had as many as 250 bakingreleases prior to sticking.

In accordance with this invention, mixtures of organic bread releasingfilm materials may be used in the coating of the meta-stable oxide oftin coating with a stable, high temperature, organic bread releasingfilm prior to the heating treatment in which the meta-stable tin oxideis converted to the tenaciously adherent green oxide. Various modes ofapplication of the film may be used. The film may be applied by any oneof various means, including dipping, spraying or brushing with asolution or colloidal dispersion of the organic film material. In thecase of the film material, such as polyorgano siloxanes and polyorganosilanes, solutions of the polymers in organic solvents may be used.Suitable organic solvents include aromatic hydrocarbons, such astoluene, xylene and coal tar naphthas; ether, such as dibutyl" ether;esters, such as amyl acetate; ketones, such as methyl isobutyl ketone;and chlorinated hydrocarbons, such as trichloroethylene. Illustrativesolvents for allyl polymers are described above.

In the case where the organic film to be applied is a polymer of afluorine-containing derivative of ethylene, aqueous dispersions are usedas mentioned above.

The concentrations of the organic film materials in the organic solventsor in the aqueous dispersions can be varied widely, for example betweenand 70% by weight of the solution, the optimum concentration dependingon the polymer and whether or not an organic solution or aqueousdispersion is used. In the case of silicones and polyorganosilanes, itis preferred that the concentration of the polymer be at least about andthe maximum about 50%, whereas in the case of the polymer offluorine-containing derivatives of ethylene it is preferred that theconcentration of the polymer in the aqueous dispersion be relativelyhigh, for example 40 to 60%. In using the allyl resin as the filmmaterial, it is preferred that the allyl resin constitute about 30 to50%. All of these concentrations are percent by weight. It is preferredthat a final organic film thickness of at least five (5) microinches beprovided.

The material containing the organic polymer to provide the breadreleasing film is applied to the meta-stable oxide of tin coating by anyof the methods described above, and then the organic solvent or water ofthe dispersion, as the case may be, can be allowed to evaporate at roomtemperature, at least to a partial extent, followed by heating theresulting coated material to convert the meta-stable tin oxide to thegreen oxide of tin. This high temperature treatment is at least about150 C., but preferably about 190 to 230 C., which will cure the organicpolymers in most of the examples described above. In the case of thepolymer of the fluorine-containing derivatives, treatment is completedat a considerably higher temperature in order to sinter the particles ofthe polymer on the surface to provide a smooth film. For example, in thecase of polytetrafluoroethylene, the final temperature used is atemperature of about 350 C. Of course, in the case of films of thepolymers of the fluorine-containing derivatives of ethylene, the finaltemperature is above the melting point of tin, so that .any tinremaining below the green tin oxide alloys with :the base metal. Thisdoes not impair the suitability of the product, because the presence ofthe organic film provides protection from corrosion of the base metal aswell as providing a film which satisfactorily releases the bakeproducts, such as bread, when the article of this invention is used inbaking.

In an embodiment of this aspect of the invention it is thus unnecessaryto insure that the original tin plated sheet is anodized so as to leavea portion of the tin un- .converted. This has the advantage that theentire tin ;-plate or coating can be completely converted by anodiz- 10ing to the meta-stable tin oxide, which is then converted to the,tenaciously adherent coating of green tin oxide for maximum utilizationof the heat absorbing prop-. erty provided by the green oxide of tin.

The articles obtained by the process of this invention providesatisfactory release of the bake goods, so that the articles can be useda great number of times before renewing the organic releasing film; atthe same time, the articles are corrosion resistant during such use, andmake possible a uniformity of product in the baking operation thatenhances the salability of the baked product.

As many possible embodiments may be made of our invention and as manychanges may be made in the embodiments hereinbefore set forth, it is tobe understood that all matter described herein, is to be interpreted asillustrative and not as a limitation.

This application is a continuation-in-part of co-pending applicationSerial No. 156,672, filed April 18, 1950.

We claim as our invention:

1. A method of making a composite article comprising a composition oftin layer bonded to a metallic supporting base having surfaces outwardlydisposed thereof, comprising the steps of subjecting the supporting basewith metallic tin coating to anodic electrolytic oxidation in anelectrolyte to form a tin oxide coating, removing the anodized articlefrom the electrolyte, coating said oxide of tin coating with a stablehigh temperature organic releasing film, and then subjecting the coatedarticle to an elevated temperature of about C. to 400 C. in order toconvert the oxide of tin to the green oxide of tin while curing theorganic film.

2. A method of making a composite article comprising a composition oftin layer bonded to a metallic supporting base having surfaces outwardlydisposed thereof, comprising the steps of subjecting the supporting basewith metallic tin coating to anodic electrolytic oxidation in anelectrolyte to form a tin oxide coating, removing the anodized articlefrom the electrolyte, coating said oxide of tin coating with a stablehigh temperature organic releasing film, and then subjecting the coatedarticle to an elevated temperature of about 150 C. to 230 C. in order toconvert the oxide of tin to the green oxide of tin while curing theorganic film.

3. A method of making a baking pan article comprising a composition oftin layer bonded to a metallic supporting base having surfaces outwardlydisposed thereof, comprising the steps of subjecting the supporting basewith metallic tin coating to anodic electrolytic oxidation in anelectrolyte to form a tin oxide coating, removing the anodized articlefrom the electrolyte, coating a portion of said oxide of tin coatingwith a stable high temperature organic releasing film, and thensubjecting the coated article to an elevated temperature of about C. to230 C. in order to convert the oxide of tin to the green oxide of tinwhile curing the organic film.

4. The method of claim 3 in which the organic releasing film contains apolyorganosiloxane.

5. The method of claim 4 in which the polyorganosiloxane is a methylphenyl silicone.

6. The method of claim 3 in which the organic releasing film containspolytetrafiuoroethylene.

7. The method of claim 3 in which the organic releasing film containspolytrifluoromonochloroethylene.

8. A method of making a baking pan article comprising a composition oftin layer bonded to a metallic supporting base having surfaces outwardlydisposed thereof, comprising the steps of subjecting the supporting basewith metallic tin coating to anodic electrolytic oxidation in anelectrolyte to form a tin oxide coating, removing the anodized articlefrom the electrolyte, fabricating said anodized article into a shapedarticle, coating a portion of said oxide of tin coating with a stablehigh temperature organic releasing film, and then subjecting the fabricated coated article to an elevated temperature of about 190 C. to 230C. in order to convert the oxide of tin

1. A METHOD OF MAKING A COMPOSITE ARTICLE COMPRISING A COMPOSITION OFTIN LAYER BONDED TO A METALLIC SUPPORTING BASE HAVING SURFACES OUTWARDLYDISPOSED THEREOF, COMPRISING THE STEPS OF SUBJECTING THE SUPPORTING BASEWITH METALLIC TIN COATING TO ANODIC ELECTROLYTIC OXIDATION IN ANELECTROLYTE TO FORM A TIN OXIDE COATING, REMOVING THE ANODIZED ARTICLEFROM THE ELECTROLYTE, COATING SAID OXIDE OF TIN COATING WITH A STABLEHIGH TEMPERATURE ORGANIC RELEASING FILM, AND THEN SUBJECTING THE COATEDARTICLE TO AN ELEVATED TEMPERATURE OF ABOUT 150* C. TO 400* C. IN ORDERTO CONVERT THE OXIDE OF TIN TO THE GREEN OXIDE OF TIN WHILE CURING THEORGANIC FILM.